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Range-based for Statement (C++)

Executes statement repeatedly and sequentially for each element in expression.

for ( for-range-declaration : expression ) 
   statement

Use the range-based for statement to construct loops that must execute through a "range", which is defined as anything that you can iterate through—for example, std::vector, or any other STL sequence whose range is defined by a begin() and end(). The name that is declared in the for-range-declaration portion is local to the for statement and cannot be re-declared in expression or statement. Note that the auto keyword is preferred in the for-range-declaration portion of the statement.

This code shows how to use ranged for loops to iterate through an array and a vector:

// range-based-for.cpp
// compile by using: cl /EHsc /nologo /W4
#include <iostream>
#include <vector>
using namespace std;

int main() 
{
    // Basic 10-element integer array.
    int x[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    // Range-based for loop to iterate through the array.
    for( int y : x ) { // Access by value using a copy declared as a specific type. 
                       // Not preferred.
        cout << y << " ";
    }
    cout << endl;

    // The auto keyword causes type inference to be used. Preferred.

    for( auto y : x ) { // Copy of 'x', almost always undesirable
        cout << y << " ";
    }
    cout << endl;

    for( auto &y : x ) { // Type inference by reference.
        // Observes and/or modifies in-place. Preferred when modify is needed.
        cout << y << " ";
    }
    cout << endl;

    for( const auto &y : x ) { // Type inference by reference.
        // Observes in-place. Preferred when no modify is needed.
        cout << y << " ";
    }
    cout << endl;
    cout << "end of integer array test" << endl;
    cout << endl;

    // Create a vector object that contains 10 elements.
    vector<double> v;
    for (int i = 0; i < 10; ++i) {
        v.push_back(i + 0.14159);
    }

    // Range-based for loop to iterate through the vector, observing in-place.
    for( const auto &j : v ) {
        cout << j << " ";
    }
    cout << endl;
    cout << "end of vector test" << endl;
}

Here is the output:

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

end of integer array test

0.14159 1.14159 2.14159 3.14159 4.14159 5.14159 6.14159 7.14159 8.14159 9.14159

end of vector test

A range-based for loop terminates when one of these in statement is executed: a break, return, or goto to a labeled statement outside the range-based for loop. A continue statement in a range-based for loop terminates only the current iteration.

Keep in mind these facts about range-based for:

  • Automatically recognizes arrays.

  • Recognizes containers that have .begin() and .end().

  • Uses argument-dependent lookup begin() and end() for anything else.

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